Format: .formatter.exs with import_deps results in errors

*This issue is closely related to https://github.com/elixir-lang/elixir/issues/7327.*

# Current behaviour

Let's assume I'm in a project with a `.formatter.exs` file looking like this:

```elixir
[
import_deps: [:some_dependency_with_formatting_rules],
inputs: [
# Some files here ...
],
locals_without_parens: [
# Some configurations ...
]
]
```

Now I'm working on a file in `lib/my_cool_module.ex` and then execute `elixir-format`. This results in the following error:

```
** (Mix) Unknown dependency :some_dependency_with_formatting_rules given to :import_deps in the formatter configuration. The dependency is not listed in your mix.exs for environment :dev
```

The dependency is available in `dev` and included in the root `mix.exs`. The issue stems from the fact that `mix format` tries find the dependencies in the current folder - which is the buffer's `default-directory` - instead of looking for them in the `.formatter.exs` folder. Due to that it's unable to find the dependencies and raises this error.

# Possible fix

Due to `mix format` looking for a `.formatter.exs` file in the current folder by default, it might make sense to allow the user to control the execution directory for `elixir-format`, this would also make a `--dot-formatter` configuration obsolete.

In addition to that a sensible default for this folder could be the projectile root folder, as this usually contains the `.formatter.exs`.

## Alternative fix

This could be fixed on the mix level.

I've actually submitted https://github.com/elixir-lang/elixir/pull/7328 a while ago, which would have fixed this issue. Sadly it got rejected due to having to cd into the `.formatter.exs` folder when loading the dependencies.

Instead it was decided that `mix format` should raise a more "descriptive" error, which you can see above.

# Workaround

As a workaround I point my `elixir-format-elixir-path` to a bash script with the following content:

```bash
if git rev-parse --is-inside-work-tree 1>/dev/null 2>/dev/null; then
cd "$(git rev-parse --absolute-git-dir | xargs dirname)"
fi

/path/to/my/elixir $@
```

This script checks if I'm in a git repository and then `cd`s into the root folder of the repository. It basically is a hacked version of the suggested fix above.